Method for separating a machining suspension into fractions

ABSTRACT

The present invention relates to a method of separating a machining suspension into fractions including abrasive grains and attrition material, such as a suspension produced in the course of the mechanical machining of silicon, quartz or ceramic material, from a cutting fluid in which abrasive grains and the attrition material are dispersed that originates from the machined material and from the machining equipment. In the method, the suspension is separated by means of wet classification, with the liquid used for the sizing process being selected in such a way that it can be mixed with the used cutting fluid and with the mixture so formed, which contains the attrition material originating from the machined material in particular, forming a stable suspension.

FIELD OF APPLICATION

The present invention relates to a method of separating a machiningsuspension into fractions, particularly of the type accumulating whensilicon, quartz or ceramic materials are machined. A used machiningsuspension consists, as a rule, of a cutting fluid in which abrasivegrains or attrition material of a cutting tool or a machining equipment,respectively, or of the machined material are dispersed.

Such machining suspensions are used, for example, in the manufacture ofthin wafers of silicon or quartz for the semiconductor or solartechnology, wherein the thin wafers are separated from mono-crystallinerods or cast blocks by application of the separating lapping technique.In one embodiment of this technique, a wire having a length of severalkilometers is passed over a coil system in such a way that a wirenetting will be created, which includes up to several hundreds of wiresin side-by-side relationship. With the addition of the machiningsuspension, it is hence possible to saw several hundreds of thin wafersfrom a block or rod of appropriate length at the same time, in a singleoperation. The used machining suspensions are composed of a cuttingfluid—preferably polyvalent alcohols or highly refined mineral oils withadditives—with abrasive grains, preferably of silicon carbide (SiC) withdifferent particle sizes, suspended therein. In the course of themechanical machining of the silicon or quartz, the abrasive grainscarried in the machining fluid is conveyed to and compressed at the sitewhere it is to produce its machining effects, by means of the machiningtools such as band saws, sawing wires or lapping disks.

The suspension used for the machining process, which carries theaccumulating attrition material, is collected and recycled into themachining process again. As a matter of fact, however, the efficiency ofthe machining suspension reduces as the application period prolongs sothat it must be replaced in the end because the given aim of themachining operation can no longer be achieved. The reason for thisresides not only in the wear of the SiC material but mainly also in theincreasing percentage of the fine particles of the silicon or quartzattrition material and in the iron attrition material from the machiningtools. The range of a typical composition of such used suspensions isillustrated in Table 1.

TABLE 1 Typical composition of machining suspensions from the process ofsilicon wafer sawing Component Percentage (mass fraction) Siliconcarbide 20-70 Cutting fluid 20-70 Silicon  0-20 Metal attrition material 0-10

PRIOR ART

For processing a used machining suspension, only some few methods areknown at present so that in practical operation the machiningsuspensions must frequently be disposed of as hazardous waste forincineration.

As a matter of fact, there are various possibilities available toincrease the period of service, i.e. the number of the possible cuts tobe performed per kilogram of suspension. To this end, for example, onecan replace a partial volume of the used machining suspension after eachcutting operation by a new machining suspension, or separate one part ofthe attrition material from the suspension by decanting. In all thesemethods, however, the used machining suspension must be disposed ofafter a more or less high number of cutting operations. As a rule, thisis performed by way of disposal as hazardous waste by incineration withsubsequent dumping of the residues.

As the costs of providing the machining suspension account for asubstantial part of the total costs incurred by the mechanical machiningof silicon, quartz or ceramic material, this widespread practicalapproach in disposal is not economic.

A method is known from the Patent Abstracts of Japan, in relation to JP1-316170, wherein used machining suspensions are centrifuged and theabrasive or grinding grains, respectively, so separated are re-dispersedin fresh cutting fluid. It is a disadvantage of this method, however,that a high fraction of attrition material and of used cutting fluidremains still in the separated abrasive grains. This method doestherefore not permit a distinct separation between the abrasive grainsand the attrition material so that the possibilities of recycling theconstituents of the machining suspension are very strongly restricted,too.

The European Patent EP 0916463A1 discloses a further method forseparation of a machining suspension into fractions. In that method, thesuspension is initially disintegrated by a distilling drying step into asolids component and a liquid component. Then a fractionating step,particularly a screening or air sizing technique, is applied forseparating the dry solids component into a fine attrition fraction and acoarse abrasive grain fraction. Subsequently to this dry sizing step,the fraction containing the abrasive grains may be used again in amachining suspension. What is a disadvantage in the drying step,however, is the fact that the additives difficult to volatilize incutting fluid remain in the solids component. Moreover, even thepreferred vacuum operation for the successful drying of the solidmaterial requires temperatures that may be detrimental to the cuttingfluid in terms of disintegration. The cutting fluid material is thenprecluded from re-use. As a result of the subsequent dry air sizingtechnique, moreover undesirable agglomerates consisting of extremelyfine particles may be transferred into the re-used coarse fraction. Thereason for this are residues of the cutting fluids that fuse theparticles together.

A strict separation between the fractions contained in the machiningsuspensions can therefore not be achieved either with this method.Successful separation into fractions is, however, the prerequisite for ahigh-quality salvage and for re-use of the components. Particularly inthe case of an intended salvage of used SiC material in the field ofceramics or abrasive grain manufacture, a strict isolation of siliconattrition material from the metal attrition material is required. Withthe known methods according to prior art, such a separation could so farnot be achieved.

In another known method according to the U.S. Pat. No. 3,9978,359 forthe separation of individual components from a cutting fluid, theseparation of the attrition material from the abrasive grains is carriedout only after a separation of the cutting fluid in several washing andrinsing operations with a subsequent drying step so that theaforedescribed problems may occur.

The European Patent EP 0 786 317 A2 discloses a method of processingused machining fluids of the claimed general type, such as cuttingfluids; this method serves to separate the abrasive grains from themachining liquid. In that method, initially water is added to the oilymachining liquid in order to reduce the viscosity of the machiningliquid by the formation of an oil/water emulsion. Then, in a wet sizingstep, the abrasive grains are separated from the liquid phase consistingof oil, water and attrition material. With such a technique, however,the intended strict separation of the abrasive grains from the attritionmaterial and the cutting fluid is not achieved either.

The present invention is now based on the problem of providing a methodof separating a machining suspension into fractions, by means of whichthe abrasive grains can be reliably and precisely separated from theattrition material and the cutting fluid so as to permit the successfulre-use of the individual components of the machining suspension.

SUMMARY OF THE INVENTION

The problem is solved with the method according to claim 1. Expedientembodiments and improvements of the method are the subject matters ofthe dependent claims.

In the inventive method, initially a liquid processing aid is selectedfor the reduction of the viscosity as a function of the respectivecutting fluid used and of the attrition material, which processing aidcan be mixed with the cutting fluid (i.e. is miscible therewith), whichmixture in its turn forms a suspension together with the attritionmaterial. Those skilled in the art are hence able to select from thegroup of processing aids miscible with the cutting fluid those materialsthat form such a mixture or suspension, respectively. A great number ofliquids known as extraction agents are available to this end.

The cutting suspension is finally separated, with application of theselected processing aid, in a wet sizing installation, for example in acounter-current classifier, where the abrasive grains can be drawn offas coarse fraction and a large volume of the attrition material issuspended in the mixture of cutting fluid and processing aid and can bedrawn off as fine-particle fraction.

In this method, the selection of a liquid processing aid plays animportant role, whose mixture with the cutting fluid constitutes astable suspension with the attrition material but not with the abrasivegrains. In conventional methods for the extractive separation of cuttingfluids from used suspensions, waste material and deposits or sludge, theselection of the solvent has so far been determined merely by itscapacity and selectivity relative to the cutting fluid to be dissolved.

For the inventive method, by contrast, the property is utilised thatsome mixtures of cutting fluid and processing aid form stablesuspensions with the attrition material originating from the machinedmaterial—in the case of silicon processing, hence the silicon particles.The term “mixture” is preferably to be understood here in the sense of amono-phase system (homogeneous mixture). After the addition ofprocessing aids, particularly solvents miscible with the cutting fluid,and after mixing the same with the machining suspension, it isnoticeable that due to the reduction of the viscosity a separation ofthe suspension into a sediment and supematant takes place, with thecutting fluid being largely transferred into the supernatant.

It was a surprise to find that certain mixtures of processing aid andcutting fluid form a stable suspension with the attrition materialcontained in the machining suspension, such as the silicon attritionmaterial, whilst other mixtures do not behave in this manner. The stablesuspensions so formed persist over several weeks, without achievement ofa complete sedimentation of the attrition material.

This effect of suspension of the attrition material from the machinedmaterial into the mixture of a processing aid with the cutting fluid isutilised in an expedient manner in the inventive method for an efficientseparation of the attrition material originating from the processedmaterial from the abrasive grains, in combination with a wet sizingoperation. The required volume of the liquid processing aid in the wetclassifier for an efficient separation of the cutting fluid and theattrition material, which originates from the machined material, fromthe fraction of sized abrasive grains is thus reduced to a minimum.

A processing aid suitable for the inventive method, preferably asolvent, hence transforms both the cutting fluid and the attritionmaterial originating from the machined material into the supernatant ina single-step immersion extraction step already, whilst it leaves theabrasive grains in the sediment approximately completely. With a smallvolume of solvent only, in combination with a wet classifier, hence acomplete separation of the attrition material, which originates from themachined material, from the fraction of the abrasive grains can besuccessfully achieved. The abrasive grain fraction can be drawn off ascoarse fraction, and metal particles can be removed therefrom, ifnecessary, by further sizing and/or by magnetic deposition for beingrecycled into the process again after separation of the solvent.

In the event that the shape of the grains in the abrasive grain fractionshould no longer permit the application as abrasive grain material,particularly when sharp cutting edges are missing after multiple use,salvage of the material is possible in a different field due to theperformed clean separation between the abrasive grains, the cuttingfluid, the attrition material and the metal. In this manner, forabrasive grains consisting of SiC, the application as expensive rawmaterial for industrial ceramics or for special abrasive bodies offersitself. Such a salvage for SiC fractions from machining suspensionscannot be realised with the processing methods according to prior art,due to the insufficient separation.

An essential advantage of the inventive method consists in thecapability of the abrasive grains to be cleanly separated from both thecutting fluid and the attrition material. By contrast against allmethods so far known, it is possible with the present method to achievea clear separation of machining suspensions that can no longer beusefully employed for machining silicon, quartz or ceramic material,into the substance fractions of abrasive grains, machined material,metal attrition material and machining fluid. Only this separation intofractions is the prerequisite for salvage of the substances.

As a rule, all the fractions of the used machining suspension can beexpediently salvaged, which are obtained in a form separated with theinventive method.

In the case of cutting fluids on the basis of mineral oil, examples ofprocessing aids presenting the properties necessary for the realisationof the inventive method are lipophilic solvents, preferably hexane,heptane, or similar substances. In the case of cutting fluids on thebasis of polyvalent alcohols, amphiphilic solvents, preferably acetoneor the like, form a stable suspension that contains predominantly theattrition material originating from the machined material.

Counter-current classifiers or centrifugal classifiers are preferablyused as wet sizing or classifying equipment.

In an expedient improvement of the method, the wet classifying step canbee supplemented by a deposition of Fe-metals, which step is preferablycarried out with a known device for magnetic metal deposition in liquidsystems. This device may be integrated into the classifier and may alsobe attached outside the classifier for purifying a drawn-off fraction.With such additional treatment, it is possible to achieve a purity forSiC as abrasive grain, which permits the salvage of SiC both in theoriginal process and for industrial ceramics and abrasive bodies.

The suspension of the mixture of the processing aid and the cuttingfluid, which is obtained from the wet classifier, may also be separatedby methods known per se. For example, mechanical separation processessuch as filtration, separation or combinations of these methods can becarried out in order to separate the solids from the liquid. It isequally possible, however, to separate initially the processing aid orthe solvent, respectively, e.g. by a distilling technique, and to mixthen the remaining mixture of cutting fluid and attrition material withanother solvent that forms a mixture with the cutting fluid but not asuspension with the attrition particles. In the case of a cutting fluidon the basis of mineral oil, an amphiphilic solvent such as acetone isappropriate to this end. In such a case, the attrition material is veryeasy to separate mechanically.

Due to a pre-treatment of the cutting fluid, a variation of theaforedescribed method may result in a reduction of the cutting fluidlevel. To this end, the machining suspension is initially treated with asolvent that does not form a stable suspension with the attritionmaterial from the machined material and the cutting fluid. The mixtureof cutting fluid and solvent so obtained can then be separated againafter a simple filtration by distillation in order to obtain the cuttingliquid and the solvent.

The inventive method can be applied to all known machining suspensionsfor the mechanical machining of brittle materials such as silicon,quartz or ceramics, in which contain the abrasive grains in dispersedform in a cutting fluid.

The method is also applicable, however, to machining suspensions arisingin the mechanical machining of other materials. Whilst a separation offerromagnetic materials does not give rise to problems, as a rule, theparticular advantage of the inventive method mainly resides in thefeature that non-ferromagnetic materials can be easily separated fromthe abrasive grains. It is irrelevant, as a matter of course, whetherthe attrition material originates from the machined material or from themachining tool.

The used machining suspensions occurring in the principal field ofapplication of the method and produced in the course of mechanicalmachining of silicon, quartz or ceramic materials consist of the cuttingfluid in which the abrasive grains as well as the attrition material aredispersed, which originates from the machined material and from themachining tool, e.g. a wire saw. Here, hard-solid particles, e.g. ofaluminium oxide, silicon or boron carbide, are used whose mean graindiameter is preferably within the range from 3 to 50 μm.

DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION

The following is a description of an example of the separation of amachining suspension into fractions, which is produced in the operationof separating lapping of silicon. The machining suspension present afterthe separating lapping operation contains, in the present example, massfractions of roughly 55% SiC as abrasive grain material, roughly 30% ofhighly refined mineral oil as cutting fluid, roughly 12% of siliconattrition material and roughly 3% of Fe metal attrition material.

The machining suspension is supplied into a counter-current classifierthat is operated with hexane as processing aid appropriate for thissystem. The ratio between the suspension and hexane amounts to 1:2approximately. An abrasive grain fraction is drawn off as coarsefraction which consists of SiC by 95%.

The separation between the silicon attrition material and the abrasivegrain consisting of SiC is so good already in the wet classifying stepthat further steps are not necessary for separating these components. Inthis example, the abrasive grain fraction contains a level of siliconattrition material, which is as low as less than 2%.

Fe metal particles are eliminated from the coarse abrasive grainfraction in the known manner by magnetic deposition. After such atreatment, the SiC level amounts to roughly 98%.

What is claimed is:
 1. Method of separating a machining suspension intofractions, which consists of a cutting fluid in which at least abrasivegrains as well as attrition material are dispersed, comprising thefollowing steps: wet classification of said machining suspension byapplication of a liquid processing aid, said liquid processing aid beingselected to form a mixture with said cutting fluid, which mixture formsa stable suspension with said attrition material persisting over aplurality of weeks without complete sedimentation of the attritionmaterial; discharge of one or several fractions containing abrasivegrains and of cutting fluid and said processing aid wherein metalattrition material from a machining tool is present in said separatedfraction or fractions containing abrasive grains; subjecting saidfractions containing abrasive grains and metal attrition material topost-treatment by a magnetic separating method for separation of themetal attrition material; and drying said separated fraction(s)containing abrasive grains and metal attrition material and condensingany liquid processing aid evaporated during the drying operation. 2.Method according to claim 1, wherein when a cutting fluid formed ofalcohols is used, an amphiphilic liquid is employed as liquid processingaid.
 3. Method according to claim 1, wherein when a cutting fluid ispresent which is formed on the basis of hydrocarbons, a lipophilicliquid is used as liquid processing aid.
 4. Method according to claim 1,2 or 3, wherein one or several fractions are discharged in said wetclassification step which contain metal attrition material from amachining tool.
 5. Method according to claim 1, wherein solids aremechanically separated from the mixture of cutting fluid and processingaid with the attrition material suspended therein.
 6. Method accordingto claim 1, wherein the liquid processing aid is separated from saidcutting fluid by a distilling method and is recycled into said wetclassification step.
 7. Method according to claim 1, wherein the wetclassification step is carried out by a counter-current or centrifugaltechnique.
 8. Method according to claim 1, wherein for a reduction ofthe cutting fluid level, the machining suspension is mixed initially,prior to wet classification, with a solvent that is mixed with saidcutting fluid, which mixtures does not form a suspension with theattrition material originating from the machined material, with themixture then being partly filtered whilst said solvent is separatedagain from the machining suspension by distillation.